1. Field of the Invention
The present invention relates to intelligent controllers, and in particular, intelligent controllers using neural network-based fuzzy logic.
2. Description of the Related Art
Uses of intelligent controllers have become more numerous and varied in keeping pace with the numerous and varied control requirements of complex modern electronic systems. For example, intelligent controllers are being called upon more frequently for use in assisting or use as servomechanism controllers, as discussed in commonly assigned U.S. patent applications Ser. No. 07/967,992, now U.S. Pat. No. 5,471,381, entitled "Intelligent Servomechanism Controller Using a Neural Network", and Ser. No. 07/859,328, now U.S. Pat. No. 5,448,681, entitled "Intelligent Controller With Neural Network and Reinforcement Learning" (the disclosures of which are each incorporated herein by reference). Further applications include control systems for robotic mechanisms.
One type of intelligent controller seeing increased use and wider application uses "approximate reasoning", and in particular, fuzzy logic. Fuzzy logic, initially developed in the 1960s (see L.A. Zadeh et al., "Fuzzy Sets and Applications", Selected Papers of L.A. Zadeh, by R.R. Yager, S. Ouchinnikov et al. (Eds.), John Wiley & Sons, 1987), has proven to be very successful in solving problems in many control applications where conventional model-based (mathematical modeling of the system to be controlled) approaches are very difficult, inefficient or costly to implement.
An intelligent controller based upon fuzzy logic design has several advantages including simplicity and ease of design. However, fuzzy logic design does have a number of disadvantages as well. As the control system complexity increases, it quickly becomes more difficult to determine the right set of rules and membership functions to accurately describe system behavior. Further, particularly in a feed-forward system, no recurrent information is embedded. In other words, conventional fuzzy logic rules retain no information about prior results or decisions. Hence, the ability to describe system behavior is limited.
The application of neural networks to learn system behavior has been suggested to overcome some of the problems associated with fuzzy logic-based designs. Using a system's input and output data, a neural network can learn the system behavior and, accordingly, generate fuzzy logic rules. See e.g.: E. Khan et al., "NeuFuz: Neural Network Based Fuzzy Logic Design Algorithms", FUZZ-IEEE'93 Proceedings, Vol. 1, pp. 647-54 (Mar. 28-Apr. 1, 1993); E. Khan, "Neural Network Based Algorithms For Rule Evaluation and Defuzzification In Fuzzy Logic Design", WCNN'93 Proceedings, Vol. 2, pp. 31-38 (Jul. 11-15, 1993); E. Khan, "NeuFuz: An Intelligent Combination of Fuzzy Logic With Neural Nets", IJCNN'93 Proceedings, Vol. 3, pp. 2945-50 (Oct. 25-29, 1993); B. Kosko, "Neural Nets and Fuzzy Systems", Prentice Hall 1992; J. Nie et al., "Fuzzy Reasoning Implemented By Neural Networks", Proceedings of IJCNN92 (International Joint Conference on Neural Networks, June 1992), pp. II702-07; and J. Buckley et al., "On the Equivalent of Neural Networks and Fuzzy Logic", Proceedings of IJCNN92, pp. II691-95.
However, a neural network may not always be the most effective way to implement an intelligent controller, since implementation of a neural network is more costly compared to fuzzy logic implementations. For example, fuzzy logic may be more effective for a particular application and, by proper programming, a conventional embedded controller can be used to implement the fuzzy logic. A neural network implementation by programming of the conventional embedded controller is also possible, but it will typically be significantly slower. Furthermore, a dedicated hardware implementation, generally more desirable, is more common for fuzzy logic than for a neural network, particularly when considering the relative costs of each.
Another problem with a neural network-based solution, particularly in a feed-forward system, is its dependence upon the present state of the input information or data. Difficulties arise when a solution requires memory in such applications as pattern recognition (including speech and handwriting), seismic signal processing, language processing, and spatiotemporal signal processing. For such applications, the outputs are not only the functions of the present inputs but also of the previous inputs and/or outputs as well.
Accordingly, it would be desirable to have an improved technique for applying neural network design to the design and implementation of fuzzy logic. In particular, it would be desireable to have a neural network-based, fuzzy logic design in which prior information could be retained for context-sensitive processing such as that needed for spatiotemporal signals. Further, it would be desirable to have an improved fuzzy logic design in which antecedent processing, rule evaluation (fuzzy inferencing) and defuzzification can be performed upon control signals generated in accordance with such neural network-based fuzzy logic design.